A piercing machine is used to manufacture a seamless steel pipe according to the Mannesmann process. The piercing machine includes a pair of inclined rolls and a piercing plug. The piercing plug is disposed on a pass line between the pair of inclined rolls. The piercing machine presses the plug into the billet while rotating the billet in the circumferential direction using the inclined rolls to pierce and roll the billet, thereby manufacturing a hollow tube stock.
The piercing machine pierces and rolls the billet heated at a high temperature. Therefore, the piercing plug pressed into the billet is exposed to the high temperature and receives a high pressure. Therefore, the piercing plug is likely to be subjected to erosion and seizure.
Generally, oxidized scale is formed on the surface of a base material of the piercing plug. The oxidized scale blocks heat from the billet and suppresses the generation of erosion. Furthermore, the oxidized scale suppresses the generation of seizure.
However, the oxidized scale wears when the billet is pierced and rolled. When the oxidized scale is removed, the temperature of the base material of the piercing plug is increased, and thus the piercing plug is eroded.
In order to increase the usage count of the piercing plug, not only forming oxidized scale on the surface of the base material of the piercing plug but also adjusting the component composition of the base material is suggested (for example, refer to Patent Documents 1 to 3 as follows).